1. Field of the Invention
This invention relates to an apparatus for measuring the length of cable either unwound from or wound on a drum or winch. The invention may find a particular application in measuring the depth of a logging sonde attached to one end of a cable and lowered in a borehole traversing earth formation, such sonde being designed to carry out measurements relative to the physical/chemical characteristics of the earth formation surrounding the borehole and/or the content of the borehole itself.
2. The Related Art
It is of the highest importance to be able to determine with reliability and accuracy the position of a logging sonde in a borehole, in order to match the physical-chemical characteristics, derived from the measurements carried out by the logging sonde, with the geological layer or the borehole location of interest, both located at a given depth. This depth determination is all the more critical in cases, which appear to be more and more common, where different logging sondes are run in the borehole at different times, and the different measurements are then merged for processing by computer. In such merging and processing, it is important that measurements obtained from a given point in a borehole on one run be merged accurately with all additional measurements obtained from the same depth point on other runs.
The determination of the depth of a sonde in the borehole usually involves two methods. The first method is based on markers, usually magnetic, placed on the cable at known depths or intervals and under known reference conditions. However, the measurements derived from these markers may be inaccurate due e.g. to an undetected permanent (plastic) cable elongation.
According to the second depth measuring method, to which is directed the present invention, one detects and counts the revolution of one, and preferably two calibrated wheels (hereafter called "encoding wheels") bearing tangentially against the cable. The two wheels are disposed on either side of the cable, in the same plane. A general over view of such system can be found e.g. in U.S. Pat. No. 4,179,817 to J. C. Lavigne and G. Segeral, assigned to the assignee of the present application.
This kind of measuring system is subjected to several sources of error and varying conditions which jeopardize both accuracy and repeatability. The linkage between the wheels and the associated devices and the devices themselves place a torque load on the wheels which, under adverse conditions often present at the borehole site, causes errors in the depth measurements due to slippage between the wheel and the cable. The errors resulting from slippage are all the more sizeable that slippage may exist undetected for relatively long periods.
Further, changes in the circumference of the wheel produce additional instability, uncertainty and inaccuracy to the measurements. These changes in the circumference may have various origins, such as e.g. mud buildup on the wheel, or differences in contact area and variations in the pressure between the wheel and the cable, or finally temperature changes which can be substantial on a well site. A further major cause of circumference change is the drastic wear of the periphery of the wheels due to the tangential contact area between the cable and the wheel, which results in a high stress on the wheel material.
Variations in the wheel-cable contact area are important, since this varies the distribution of the pressure between the cable and the wheel. Thus, one requirement for accurate cable measurements under well logging conditions is the placement of the encoding wheels at a point on the cable where the contact area will not vary and where contact forces are not sufficient to distort either the cable or the wheel.
In a known manner, in order to maintain the cable in a straight line along a given segment including the tangential point of contact between the wheels and the cable, there are provided two sets of guiding means, each including two rollers, and being disposed on either side of the tangential point, along the cable axis.
However, this implementation, although it allows one to define a straight cable segment, is not sufficient to fully isolate the straight cable segment from the various forces and torques to which the cable is submitted between the winch and the upstream guiding means and between the downstream guiding means and the structure erected above the well. Such forces e.g. are due to the spooling of the cable on the winch which generally causes cable path curvature and acceleration forces. Considerable lateral forces are required to deflect the cable at a certain angle from a center line. These forces are extreme when the cable is held against the flanges of the winch in order to start each new layer. Furthermore, the cable presents a "natural" curvature due to the "memory" of the cable which is for the most time wound on the winch. The straight segment of cable between the guide means transmits the forces and torques to the tangent encoding wheels, to the detriment of the contact area. This is all the more detrimental since each guiding means cannot usually be placed closer to the tangential contact point than one foot apart.
As an attempt to remedy these problems, one has proposed to provide additional guiding means placed close to and on each side of the wheel/cable tangential contact area. Each additional guide means comprises a pair of arms disposed on either side of the cable. Each arm has its end bearing against the cable configured in U-shape form, in order to define a guiding area substantially complementary to the cable. However, these arms being made of a material of low density, such as aluminum, for weight reduction purpose, wear relatively rapidly and thus shortly do not play any guiding role.
Therefore, there is a need for a reliable and accurate device for measuring the length of cable either wound on or unwound from a winch, especially in the application of logging for an accurate and reliable determination of the depth of a logging sonde lowered in a borehole.